Non-linear optical crystals, such as lithium niobate and lithium tantalate, are employed in components for laser optics, for example in frequency doublers or in frequency mixers, it being necessary for the crystal to have a high level of purity for this use. Since it is not possible to avoid a small amount of impurities being contained in the final crystal even when growth is carried out under the purest possible conditions, these crystals are also referred to as “nominally” pure crystals.
The impurities are ions which are present inside the crystal in a plurality of valances and which can act as electron donors and recipients. Its is especially problematic for the aforementioned applications is that particularly intense light, especially laser light, can cause the charge of the ions to be redistributed and thus can induce a space-charge field. In electro-optical crystals, the space-charge field causes the refractive index to be modulated in a disruptive manner, and this represents “optical damage” to the light beam insofar as the light beam fans out in the crystal and the quality thereof is therefore lowered.
In this way, iron impurities, which are present in the Fe2+ and Fe3+ valence states, cause a charge transfer, in which the Fe2+ ion releases an electron which enters the conduction band, an Fe3+ ion remaining behind and the free electron in the conduction band being captured by the Fe3+ ions which are already present. This charge transfer causes serious optical damage. In order to prevent the charge transfer and thus optical damage, attempts have been made to minimise the amount of impurities during the growth stage. However, it has proved to be impossible to date to grow crystals with a residual dopant content of less than 10 ppm.
Another option is to effectively purify the crystal at a later stage, i.e. to render the undesired dopant non-disruptive by oxidising the electron-donating ions, that is to say by converting, for example, the Fe2+ ions into the Fe3+ state. The thus released surplus electrons are then “washed out” by applying a voltage. Methods of this type are referred to as “oxidation” methods, the purifying effect thereof being supported by high temperatures and external voltages applied to the crystal. In this case, the concentration ratio V of filled to empty impurities serves as a measure for the degree of oxidation; in the case of iron, this is the concentration ratio of Fe2+ to Fe3+. If V is very low, charge transfer cannot occur since there is a lack of electron donors, and, accordingly, no space-charge field is formed and any optical damage is avoided.
Although the known methods clearly have a purifying effect, the treated crystals are not sufficiently pure for applications using high-intensity laser beams. For this reason it has rarely been possible before now to use known non-linear optical components, such as frequency doublers, frequency mixers or optical parametric oscillators (OPOs) with high-intensity laser beams. The optical damage to which these laser beams are subjected in the components impairs the functionality thereof to an unacceptable extent.